Add wasm_syscall feature to build system

config.toml.example

@@ -305,6 +305,11 @@
 # result (broken, compiling, testing) into this JSON file.
 #save-toolstates = "/path/to/toolstates.json"
 
+# Flag indicating whether `libstd` calls an imported function to hande basic IO
+# when targetting WebAssembly. Enable this to debug tests for the `wasm32-unknown-unknown`
+# target, as without this option the test output will not be captured.
+#wasm-syscall = false
+
 # =============================================================================
 # Options for specific targets
 #

src/bootstrap/check.rs

@@ -1240,6 +1240,14 @@ impl Step for Crate {
             cargo.env(format!("CARGO_TARGET_{}_RUNNER", envify(&target)),
                       build.config.nodejs.as_ref().expect("nodejs not configured"));
         } else if target.starts_with("wasm32") {
+            // Warn about running tests without the `wasm_syscall` feature enabled.
+            // The javascript shim implements the syscall interface so that test
+            // output can be correctly reported.
+            if !build.config.wasm_syscall {
+                println!("Libstd was built without `wasm_syscall` feature enabled: \
+                          test output may not be visible.");
+            }
+
             // On the wasm32-unknown-unknown target we're using LTO which is
             // incompatible with `-C prefer-dynamic`, so disable that here
             cargo.env("RUSTC_NO_PREFER_DYNAMIC", "1");

src/bootstrap/config.rs

@@ -106,6 +106,7 @@ pub struct Config {
     pub debug_jemalloc: bool,
     pub use_jemalloc: bool,
     pub backtrace: bool, // support for RUST_BACKTRACE
+    pub wasm_syscall: bool,
 
     // misc
     pub low_priority: bool,
@@ -281,6 +282,7 @@ struct Rust {
     quiet_tests: Option<bool>,
     test_miri: Option<bool>,
     save_toolstates: Option<String>,
+    wasm_syscall: Option<bool>,
 }
 
 /// TOML representation of how each build target is configured.
@@ -461,6 +463,7 @@ impl Config {
             set(&mut config.rust_dist_src, rust.dist_src);
             set(&mut config.quiet_tests, rust.quiet_tests);
             set(&mut config.test_miri, rust.test_miri);
+            set(&mut config.wasm_syscall, rust.wasm_syscall);
             config.rustc_parallel_queries = rust.experimental_parallel_queries.unwrap_or(false);
             config.rustc_default_linker = rust.default_linker.clone();
             config.musl_root = rust.musl_root.clone().map(PathBuf::from);

src/bootstrap/lib.rs

@@ -420,6 +420,9 @@ impl Build {
         if self.config.profiler {
             features.push_str(" profiler");
         }
+        if self.config.wasm_syscall {
+            features.push_str(" wasm_syscall");
+        }
         features
     }